It has been known that it is advantageous to keep oxygen dissolved by diffusing bubbles in water in order to activate bacteria for water treatment or grow/activate aquatic organisms in sewerage/aquaculture and the like. Further, even in the manufacturing industry, it has been widely known that it is advantageous to diffuse bubbles in liquid in order to promote a gas-liquid reaction.
To this end, it is preferable to reduce the size of bubbles as small as possible in order to keep bubbles in liquid longer by reducing buoyancy of the bubbles and increase reaction efficiency by increasing a surface area that comes in contact with liquid.
As a method of diffusing bubbles in water in the prior art, for example, a method of discharging compressed air from a compressor through fine holes such as diffuser tubes or a method of mixing and stirring bubbles in water by rotating a bladed wheel in the water is the most typical.
However, there was a problem in these methods in that the cost of the equipment is high and maintenance is difficult, and discharging bubbles through fine holes or mixing and stirring bubbles in water is not enough to make the bubbles microscopic, so the bubbles remain large and therefore it is difficult to generate microscopic bubbles.
Accordingly, various methods have been proposed in recent years, as follows, in order to generate microscopic bubbles:    (1) Generating microscopic bubbles by rapidly ejecting liquid from a nozzle and mix gas into the liquid. (For example, see Patent Document 1)    (2) Generating microscopic bubbles by stirring/shearing a mixture gas by generating rotational flow in liquid in a container. (For example, see Patent Document 2)    (3) Generating microscopic bubbles by making a gas-liquid mixture collide with a baffle or collision projections of a static mixer and stirring it. (For example, see Patent Document 3)    (4) Generating microscopic bubbles by dissolving gas in liquid by mixing and pressurizing the liquid and the gas, and then depressurizing the gas-liquid mixture. (For example, see Patent Document 4)
However, the methods of (1) to (3) of the prior art have the following problems:    The case (1) needs fine adjustment for achieving an optimal mixture of liquid and gas and is used only for small equipment, so it is difficult to generate a large amount of microscopic bubbles in a short time.    The case (2) needs fine adjustment for stably generating microscopic bubbles, so it has difficulty in operating for a long time without a worker. Further, a cyclone type turning mechanism having a spiral inflow channel is used, so it depends on a turning force by kinetic energy of liquid. Therefore, energy is insufficient and a sufficient turning force cannot be achieved, making it difficult to generate a large amount of microscopic bubbles in a short time.    The case (3) depends on simple collision, so bubbles cannot be made sufficiently microscopic. Further, a loss of energy due to resistance of the static mixer is large, and there is a need for power for sending a gas-liquid mixture to compensate the loss.
On the other hand, the case (4) includes a gas-liquid dissolving tank, as shown in FIG. 23, and generates microscopic bubbles by dissolving gas in contact with liquid under pressure using the tank, and then depressurizing or exposing it to atmosphere. Therefore, it requires only limited fine adjustment and is relatively easy to achieve operation control, as compared with the methods (1) to (3). However, in order to generate a large amount of bubbles in a short time, it is required to promote dissolution by increasing the contact area between the gas and the liquid in the gas-liquid dissolving tank, so the equipment of the gas-liquid dissolving tank increases in size and a high cost is required.
As a solution, there has been proposed a method of increasing a contact area of gas and liquid by making a channel in a zigzag shape, using shelf-shaped separation walls 14, 15, and 16 in a gas-liquid dissolving tank 3, as shown in FIGS. 24 to 26 (for example, see Patent Document 5), but this method still has a limit in promoting dissolution. This is because the method is supposed to make an ideal state in which gas flows through an upper portion and liquid flows through a lower portion anywhere in the channel, that is, the states shown in FIGS. 24 and 25, but actually, the ideal state cannot be maintained and the balance breaks after all, so most part of the channel is easy to submerge under the water level L, as in FIG. 26. In this case, the separation walls under the water level L have no meaning, so it is little different from the gas-liquid dissolving tank of FIG. 23. In order to prevent the submerging, it is required to control liquid to flow in an appropriate amount anywhere in the channel with a space left over the liquid by making a flow rate adjustable at predetermined portions in the channel, for example, at the openings 14m, 15m, and 16m of the separation walls, and as a result, the cost increases and a practical equipment cannot be achieved.
As described above, not only the cases (1) to (3) of the prior art, but the case (4) of the prior art has a limit in respect of both cost and performance